Common Error

be made by
be made from
be made of


These three chunks are quite confusing.

be made by = the people or company who made the thing
be made of = the substance or material, as long as this has not changed
be made from = the substance or material, when this has changed

The computer is made by Dell. CORRECT
Chocolate is made from milk and cocoa and sugar. CORRECT
Books are made of paper. CORRECT

The desk is made by wood. INCORRECT
The bottle of made from glass. INCORRECT
The cake was made of my mother. INCORRECT.

'Be made by' is easy to remember. 'Be made of' and 'be made from' are a little more tricky. My advice is to use 'be made of', not 'be made from'. 'Be made of' is usually more correct, and is used more often.